Juvenile myositis (juvenile dermatomyositis, JDM, and juvenile polymyositis, JPM)

The term "myositis" means inflammation in the muscle, and there are two forms of this process that affect children. The most common is juvenile dermatomyositis (JDM), in which the children have a characteristic rash that does not go away. Frequently children with JDM experience weakness with activities that have not been a problem before, such as getting into a car or standing up from a chair.

The second form is called juvenile polymyositis (JPM), which is the weakness without the rash. Juvenile dermatomyositis is rare, affecting 3,000 to 5,000 US children under the age of 18. Juvenile polymyositis occurs in even fewer children.

What causes juvenile myositis?

The cause of juvenile myositis has not yet been determined. However, factors thought to be associated with dermatomyositis include an increased defensive response to microbes in the immune system and damage to small blood vessels (vasculitis). These factors are seen in children with JDM with a strong family history of autoimmune problems.

Symptoms of juvenile dermatomyositis/juvenile polymyositis

The symptoms of JDM/JPM often appear gradually. At times, there may be a more acute, or intense, onset of symptoms. The following are the most common symptoms of JDM. However, each child may experience symptoms differently. Children with JPM do not have a rash.

Fever
Redness, sometimes with swelling, around the eyes, over the knuckles and finger joints, which may extend down the arms and body
Muscle weakness, sometimes with hoarseness and difficulty swallowing
Fatigue
Malaise (general discomfort)
Muscle pain and tenderness
Irritability
Weight loss
Joint pain and inflammation
Calcium deposits under the skin (calcinosis)
Mouth ulcers

Symptoms of JDM/JPM may resemble other medical conditions or problems. Always consult your child's physician for a diagnosis.

Phases of juvenile dermatomyositis/juvenile polymyositis

The course of JDM/JPM is often divided into three phases, based on symptoms and findings on examination. The four phases are briefly described below:

Prodromal period – This phase can last for weeks or months. The symptoms seen during this period are nonspecific (general symptoms which can indicate many different conditions).
Progressive muscle weakness and rash – In this phase, muscle weakness exists, and in the case of JDM, the rash can be either progressive or stable.
Remission – With the administration of appropriate therapy, remission may occur without leaving any apparent consequences. In severe cases without prompt medical attention, there may be loss of bone mass with increased risk of fracture, residual calcifications (some resolve) or loss of range of motion.

Diagnosis

In addition to a complete medical history and physical examination, diagnostic procedures for juvenile myositis may include:

Laboratory tests – to assess the presence of antibodies, muscle enzymes, and indicators of immune inflammation and vascular damage in the blood.
Magnetic resonance imaging (MRI) – a diagnostic procedure that uses a combination of large magnets, radiofrequencies, and a computer to produce detailed images of organs and structures within the body. More here about MRI.
Muscle biopsy – removal of a small piece of muscle for microscopic examination from the area of inflammation identified by MRI.
Dual energy X-ray absorptiometry (DXA) – a bone-density x-ray used to assess potential damage to bones that may be reversible if appropriate care is given
Electromyography (EMG) – an electrical test to determine nerve or muscle damage.

Treatment for juvenile myositis

Juvenile myositis has a range of disease severity. After therapy some children have no apparent residual problems. Others, who have more severe symptoms, may have some residual problems in adulthood. Employing a battery of immunosuppressive therapies and a multidisciplinary team approach to treatment, remission of the disease may be achievable in time. Treatment may include:

Medications such as glucocorticosteroids and methotrexate (to treat the inflammatory process) and hydroxychloroquine (to help treat the skin disease of dermatomyositis). Other drugs include immunosuppressants such as CellCept®, cyclosporin, cyclophosphamide, or immunoglobulin treatments to bolster the immune system. To help make the drug administration less stressful, some of the children have a temporary port placed.
Physical and occupational therapy (to improve muscle function and strength).
Liberal use of sunscreens (to prevent further irritation or damage to the skin).
Nutritional support (to ensure an adequate diet).

Social work, school and family support are also very important.